The invention relates to a hydraulic system with an adjustable hydraulic pump, whose output pressure can be controlled as a function of the pressure existing at a load sensing connection, a reservoir, at least one hydraulic device with a supply line and a valve arrangement between the pump, the reservoir and the device.
Modern agricultural tractors are today equipped with constant pressure hydraulic systems in which control valves block the flow of fluid from the pump when they are in their neutral position (closed-center-system). Furthermore, in these systems the hydraulic fluid output can be compensated in such a way that only the necessary flow of fluid is delivered by the pump (power-on-demand). The significant feature of such systems is the so-called load sensing line from the device to the load sensing connection of the hydraulic pump, by means of which the hydraulic pump adjusts its output pressure at all times as a function of the pressure existing at the load sensing connection and thereby provides only the necessary supply. This permits a considerable saving in energy.
The attached implements used in agriculture are primarily equipped with self-contained control devices for the control of their hydraulic systems. In order to avoid a costly double actuation by the tractor-borne control device and the control device on the side of the attached implement it would be advantageous if the latter could be connected directly to the tractor-borne hydraulic system without any supplementary procedures. However, since the hydraulic controls on the side of the attached implements are most frequently configured as constant flow valves, that are open in their center position (open-center-design) or as constant pressure valves that are closed in their center position (closed-center-design), they are not always provided with the necessary load sensing connection. If, for example, a control valve of the aforementioned tractor hydraulic system is used, in order to control the fluid flow to the hydraulic devices of an attached implement which requires a constant pressure supply, then the hydraulic pump of the tractor must be operated continuously at the maximum operating pressure. This results in power losses greater than necessary. Consequently, the fuel consumption also increases, and additional heat loading is imposed.
A solution to this problem could be provided by installing a hydraulic load sensing line on the attached implement, which detects the hydraulic pressure of the device on the attached implement and transmits this to the load sensing connection of the tractor-borne hydraulic pump. However, this solution requires changes to the hydraulic system of the attached implement (hydraulic circuits and valve arrangement), which can become very costly and complicated, if several devices must be considered.
Another solution requires the use of sets of valves, which are usually supplied with attached implements with constant pressure hydraulic systems in order to be able to connect these to tractors with constant flow hydraulic systems. In such a set of valves an electric control signal from the attached implement is utilized in order to control a relief valve of the set of valves. In this method of operation the operator must carefully adjust the tractor valve which delivers the hydraulic fluid, in order to minimize power losses. But, even with such an adjustment the uninterrupted fluid flow is maintained, if the device of the attached implement is not actuated, which results in undesirable power losses.